Hitherto, the following variable displacement vane pump is known. Specifically, the variable displacement vane pump includes vanes received in slot grooves of a rotor so as to be projectable therefrom and retractable therein. Pump chambers are defined by an inner circumferential surface of a cam ring, an outer circumferential surface of the rotor, and the vanes. Volumes of the pump chambers are changed by a swing of the cam ring. For example, a vane pump described in Japanese Patent Application Laid-open No. 2012-87777 includes a solenoid for applying a biasing force to a control valve for controlling an eccentricity of the cam ring so that a delivery flow rate becomes equal to a desired value. Through the application of a predetermined biasing force using the solenoid, the delivery flow rate is controlled.
In the vane pump described in Japanese Patent Application Laid-open No. 2012-87777, however, in a case where a magnitude relationship between a pressure in a first control chamber and a pressure in a second control chamber frequently changes, there is a fear in that a sealing member undesirably moves horizontally to lower durability of an edge portion of the sealing member. Further, if a volume of the first control chamber or the second control chamber is fluctuated due to the swing of a cam ring 8, the pressure is further fluctuated. Therefore, the phenomenon of the horizontal movement of the sealing member becomes further noticeable. Further, if a leakage occurs from a sealed portion in a state in which an absolute pressure in the first control chamber or the second control chamber is high, cavitation erosion occurs due to air contained in operating oil. In order to avoid the occurrence of cavitation erosion, it is conceivable to select a material having a high hardness and a high strength as a material of the sealing member. If such a material is used, however, when the sealing member moves due to a fluctuation in differential pressure between the first control chamber and the second control chamber, there is a fear in that an adapter ring on which the sealing member is provided may be struck to lower durability of the adapter ring.